Nitride strengthened steel

ABSTRACT

ALUMINUM AND NITROGEN IMPROVE THE LOAD-CARRYING ABILITY OF DECARBURIZED STEEL SHEETS AFTER STRAINING UP TO ABOUT 20% AND ANNEALING BY EXPOSURE TO ENAMEL FIRING TEMPERATURES.

y 1972 E. H. MAYER ET L NITRIDE STRENGTHENED STEEL 2 Sheets-Sheet 1 Filed Aug. 13, 1970 2.5.5 0 N 8 g 8 2 N m 0 mmfiw $2 535 52 mQE z INVENTORS r 8 r 09 m H0 mm 0 5 July 18, 1972 F1 'Led Aug. 13, 1970 YIELD STRENGTH |O p$i E. H. MAYER ETAL NITRIDE STRENGTHENED STEEL 2 Sheets-Sheet 2 SOLUBLE ALUMINUM NITROGEN RATIO INVENTORS Edward H Mayer Evan M. O/fver United States Patent O US. Cl. 148-3 4 Claims ABSTRACT OF THE DISCLOSURE Aluminum and nitrogen improve the load-carrying ability of decarburized steel sheets after straining up to about 20% and annealing by exposure to enamel firing temperatures.

CROSS REFERENCE TO RELATED APPLICATIONS This is a continuation-impart of our copending application Ser. No. 732,448, filed May 27, 1968, which is now abandoned and is itself a continuation-in-part of application Ser. No. 538,872, filed Mar. 30, 1966, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a new ferrous alloy and more particularly to new iron base alloy sheets for vitreous enameling having better post-enameled strength than the prior art enameling steels.

Simple low carbon steel sheets which have been almost completely decarburized are the preferred material for vitreous enameling because of their relative freedom from enameling defects. However, difficulties may be encountered with distortion and enamel chippage on portions of those parts which have been subjected to forming operations prior to coating with enamel. These difficulties are particularly severe when the enameled sheets are used in load-bearing applications. The failure of the brittle coating is attributed to deformation of the base material under load due to loss of strength in the areas which have been strained an amount falling within the critical strain range during the forming operations and subsequently annealed while firing the enamel. Critical strain is that amount of strain that causes excessive grain growth and consequent loss of strength while firing the enamel. In the prior art decarburized enameling steels, the critical strain range is from about 3 to 8 strain.

The principal object of this invention is to provide a ferrous base sheet for vitreous enameling having improved load-carrying ability after being strained up to about 20% and annealed by exposure to enamel firing temperatures.

Another object is to provide a ferrous sheet having improved resistance to distortion after being formed and vitreous enameled.

SUMMARY OF THE INVENTION It has been discovered that the foregoing objects can be attained by providing enameling steels containing very low carbon, e.g. 0.010 wt. percent max, with small amounts of aluminum and nitrogen. However, the ratio of said aluminum to said nitrogen must be from 1.0 to 3.0 if the steels are to be strained by 15-20% and then enameled.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the effect of aluminum and nitrogen on the yield strength of decarburized ferrous sheets which have been strained and annealed.

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FIG. 2 is a graph showing the effect of the ratio of the aluminum to the nitrogen on the yield strength of decarburized ferrous sheets which have been strained by 20% and annealed.

DESCRIPTION OF THE PREFERRED EMBODIMENT Broadly, the ferrous metal sheets of the invention consist essentially of 0.010 wt. percent max. carbon, 0.12 to 1.00 wt. percent manganese, 0.008 to 0.025 wt. percent nitrogen, 0.010 to 0. 08 wt. percent aluminum, balance 11'0I1.

By balance iron we do not wish to exclude residual impurities and incidental elements which may be present in amounts which do not substantially detract from the desired properties of the steels. For example, the steels usually contain impurities such as phosphorus up to 0.020 wt. percent and sulfur up to 0.035 wt. percent. However, if the manganese content of the steel is around 0.12 wt. percent, the sulfur should be limited to 0.015 wt. percent so that the manganese to sulfur ratio is at least 8:1, thereby preventing hot-shortness.

As used hereinafter, the term aluminum is intended to mean acid-soluble aluminum, i.e. aluminum which is not in the form of aluminum oxide and is therefore available to combine with the nitrogen in the steel.

The reasons for the limits of the above ranges are as follows. Carbon is well known to be a cause of primary boiling in enameling steels and should therefore be kept low. A good practical upper limit is 0.010 wt. percent, and the carbon will generally fall within the range of 0.001 to 0.010 wt. percent. The manganese is present primarily to prevent hot-shortness, and while the upper limit is set at 1.00 wt. percent, manganese will generally fall within the range of 0.12 to 0.50 wt. percent. A minimum of 0.008 wt. percent nitrogen has been found necessary to consistently achieve the results of the invention, while above 0.025 wt. percent nitrogen reduces drawability. In general, the upper limit of the nitrogen should be about 0.020 wt. percent. A minimum of about 0.015 wt. percent aluminum is necessary to prevent critical strain in the steel, while the steel may contain up to about 0.08 wt. percent aluminum and still have good resistance to excessive grain growth when strained up to about 15% prior to enameling.

A preferred range of compositions of the ferrous sheets of this invention is as follows:

the balance being iron and normal impurities.

The curves in FIG. 1 compare the effect of varying amounts of strain on the yield strength of the decarburized ferrous sheets of the invention with the same effect on prior art enameling steels. The steels of the invention consisted essentially of 0.003 wt. percent carbon, 0.30 wt. percent manganese, 0.010 Wt. percent nitrogen, 0.020 wt. percent aluminum, balance essentially iron, while the prior art steels consisted essentially of 0.002 wt. percent carbon, 0.22 wt. percent manganese, 0.003 wt. percent nitrogen, under 0.01 wt. percent aluminum, balance essentially iron. The percentage increase in length resulting from cold rolling samples of these sheets was used as the percent strain along the abscissa and the ordinate was the yield strength obtained after subjecting the strained specimens to a simulated enamel firing treatment consisting of heating to 1500 F. for three minutes, cooling to under 1000 F., reheating to 1500" F. for three minutes and cooling to room temperature. The inflection point on each curve is indicated by an arrow to designate the critical strain at the location where the yield strength is decreasing at the maximum rate.

In FIG. 1, curve 11 is representative of the enameling steels of the prior art and curve 12 is representative of the alloys of this invention. Comparing these two curves shows that the alloys of this invention are an improvement over the prior art because they are capable of being strained to a much greater extent Without becoming critically strained and in addition they have higher yield strength at all levels of strain.

As disclosed earlier, the decarburized steels must contain at least 0.008 Wt. percent nitrogen to prevent critical strain, at least up to 20% strain. To illustrate this point, the following Table I was prepared.

Each of these steels contained carbon and manganese within the claimed ranges.

Each of said steels was strained by 20% and then anhealed at 1460 F. for six minutes, said anneal being substantially equivalent to the simulated enamel firing treatment used to obtain the data for FIG. 1. As can be seen, each of the steels Nos. 1-3 had yield strengths in excess of 20,000 p.s.i., while each of the steels Nos. 4-6 had yield strengths of less than 20,000 p.s.i.

FIG. 2 shows the eflfect of the ratio of the acid-soluble aluminum to the nitrogen on the yield strength of decarburized ferrous sheets. The yield strength was measured after straining by 20% and annealing at 1460 F. for six minutes. As can be seen, the yield strength is above about 20,000 p.s.i. for ratios between 1.0 and 3.0, while above a ratio of 3.0 the yield strength is generally below 20,000 p.s.i. When the ratio is about 1.0 there is quite a bit of scatter in yield strength values and, in addition, the drawability of the steel is minimal. Preferably, therefore, the ratio is a minimum of 1.5; at this value there is little scatter in yield strength values and a sheet of steel having such a ratio can have a minimum total elongation of 30%. If extremely good drawability is desired, the ratio should be a minimum of 1.9; at this value a minimum total elongation of 40% can be attained.

In the preferred embodiment of the invention, a ferrous sheet should be provided consisting essentially of 0.005 wt. percent max. carbon, 0.12 to 0.50 wt. percent manganese, 0.008 to 0.015 wt. percent nitrogen, 0.012 to 0.038 wt. percent acid-soluble aluminum, balance essentially iron, the ratio of said acid-soluble aluminum to said nitrogen being within the range of 1.5 to 2.5. Such a sheet can be strained by 15-20% and will have a total elongation of at least 30% and a minimum yield strength of 30,000 p.s.i. after enameling.

It is desirable that the iron base alloys of this invention be as free as possible of normal impurities, as illustrated by the specific composition shown below:

There is nothing unusual about the form or method of adding aluminum and any of the common methods of adding nitrogen, such as through the use of nitrided manganese and/or calcium cyanamide, are satisfactory for producing the alloys of this invention.

Preferably the steel is made as follows. A rimming steel to which nitrogen has been added is poured into a mold and killed with aluminum after rimming two to six minutes. For example, a nitrogen content of about 0.014 wt. percent was obtained by adding 300 pounds of calcium cyanamide and 1500 pounds of nitrided manganese to a 350 ton heat of steel. The steel was killed in the mold by adding thereto 1.5 pounds of aluminum shot per ton of steel. The steel is slabbed, heated to above 2200" F., soaked at this temperature, hot rolled into strip above 1550 F., cooled rapidly to below 1100 F. and coiled hot. After cooling to room temperature, the strip is pickled, cold reduced about 60% and decarburized at between 1250 and 1400 F. in a moisture annealing gas containing between 18 and hydrogen.

The various examples mentioned previously are only intended to be used in an illustrative manner; the actual limitations of the alloys of this invention being included in the following claims.

In the specification all composition percentages are by weight.

We claim:

1. A sheet of ferrous metal consisting essentially of 0.001 to 0.010 wt. percent carbon, 0.12 to 1.00 wt. percent manganese, 0.008 to 0.025 wt. percent nitrogen, 0.010 to 0.075 Wt. percent acid-soluble aluminum, balance iron, the ratio of said acid-soluble aluminum to said nitrogen being within the range of 1.0 to 3.0, wherein said ferrous metal has a minimum yield strength of 20,000 p.s.i. after being strained by 20% and then annealed at 1460" F. for six minutes.

2. A product as recited in claim 1, the ratio of said acid-soluble aluminum to said nitrogen being at least 1.5, said ferrous metal having a minimum total elongation of 30% and a minimum yield strength of 20,000 p.s.i. after being strained by 20% and then annealed at 1460 F. for six minutes.

3. A product as recited in claim 1, said ratio being within the range of 1.9 to 3.0, said ferrous metal having a minimum total elongation of 40% and a minimum yield strength of 20,000 p.s.i. after being strained by 20% and then annealed at 1460 F. for six minutes.

4. A sheet of ferrous metal as recited in claim 1 consisting essentially of 0.005 wt. percent max. carbon, 0.12 to 0.50 wt. percent manganese, 0.008 to 0.015 wt. percent nitrogen, 0.012 to 0.038 wt. percent acid-soluble aluminum, balance iron, the ratio of said acid-soluble aluminum to said nitrogen being within the range of 1.5 to 2.5.

References Cited UNITED STATES PATENTS 2,597,979 4/ 1952 Darmara 148-36 X 3,163,565 12/1964 Wada 75-124 X 3,173,782 3/1965 Melloy et aI 75l23 3,239,390 3/1966 Matsukura et a1. 14812.1 3,259,488 7/1966 Nakamura 148-36 X 3,320,099 5/1967 Weber 148-2 3,357,822 12/1967 Miyoshi et al. 148-36 X OTHER REFERENCES German-Anslegeschrift 1,066,598, Oct. 8, 1959, 3 pages.

CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R. 75-l24 

